This invention relates to a cyclic process using a selectively gas permeable membrane to separate a component from a gas mixture. More specifically, it relates to a membrane separation process useful for recovery of volatile organic compounds emitted from storage tanks utilizing a membrane comprising a selectively gas permeable membrane polymer. The process includes repetitively cycling between flow and non-flow of gas through the membrane.
Liquid volatile organic compounds (xe2x80x9cVOCxe2x80x9d) are stored and dispensed from tanks. A very common example is in the field of distribution of combustion engine fuel such as gasoline for fueling automobile and aircraft engines. The storage tanks usually have large capacities, receive bulk shipments of fuel from a supply source and dispense smaller amounts in multiple events, e.g., filling individual automobile tanks at service stations. The gas space above the liquid in the tank is sometimes called the xe2x80x9cullagexe2x80x9d of the tank. Usually there is a high concentration of VOC in the ullage of fuel storage tanks.
Prior to the time that air pollution by VOC emissions became an environmental protection concern, emission controls on storage tanks were directed mainly to preventing fire and explosion hazards. Few controls were aimed at curbing fugitive emissions such as emissions of VOC incidental to dispensing fuel from bulk storage tanks and to storing the fuel in the tanks.
More recently heightened awareness has developed of the need to reduce fugitive emissions resulting from storage and dispensing of VOC. As a result vapor recovery systems for VOC with increasing degrees of sophistication have been deployed. For example, to reduce environmental emissions of VOC vapor during automotive and other types of fuel fill-ups, fuel suppliers and distributors have begun to install vapor recovery systems at fuel dispensing stations. Such systems usually have suction equipment that draws VOC vapors and air present during fuel transfer at the fuel dispensing nozzle back to the ullage of a bulk storage tank. The returning gas mixture enters the void in the tank created when the dispensed liquid leaves.
Traditionally storage tanks merely had P/V valves (Pressure-Vacuum valves) intended to maintain the tank within a range of slight positive and negative pressure, i.e., a few inches of water pressure. The returning gas mixture from dispensing operations, as well as other factors, caused pressure to build up in the ullage over time. Of course, when tank pressure exceeded the upper limit of the P/V valve, excess gas containing VOC was discharged to the environment.
Certain advanced VOC fugitive emission control systems are designed to operate with a slight negative pressure in the ullage of the bulk storage tank. That is the tank is under a vacuum relative to ambient atmosphere. Such systems offer the advantage that any leaks that occur will cause outside air to flow into the vapor recovery systems, rather than allow vapor to escape to the atmosphere. In addition to the gas buildup mentioned earlier, air in-leakage contributes to pressure increase in the tank. The liquid fuel evaporates into the incoming fresh air and the mass of the vaporized fuel plus the mass of air within the fixed ullage volume increases the pressure. Negative pressure thus can only be maintained if gas is exhausted to the environment from time to time. However, it is necessary to strip all or a portion of the VOC from the exhausted gas. Otherwise, the VOC in the discharged gas defeats the purpose of the pollution control system.
Various techniques have been proposed to remove VOC emissions from bulk storage tanks operating at subatmospheric pressure. A method gaining commercial acceptance uses a selectively gas permeable membrane to separate the VOC component from the benign air component of the ullage mixture. The non-VOC component, composed primarily of nitrogen and oxygen, is preferentially permeable through the membrane and is emitted to atmosphere substantially free of the VOC component. VOC is less permeable, largely does not pass through the membrane and is returned to the storage tank.
The membrane separation vapor recovery system is contemplated to operate cyclically and emit to atmosphere discontinuously. Emissions occur only when the tank pressure exceeds a pre-selected high pressure limit. At other times, flow through the membrane is stopped. For example, tank pressure descends below the high pressure limit as a consequence of discharging primarily non-VOC component gas to the ambient atmosphere. At a preselected low pressure limit, discharge stops. At these times, the vapor is stagnant in the separation membrane module and in the gas transfer lines immediately upstream and downstream of the module.
Although the separation membrane selectively permeates oxygen and nitrogen, it does not absolutely reject VOC compounds. Consequently, the gas that permeates the membrane and is vented to the environment includes some VOC vapor, albeit less than that which would vent had the membrane not been utilized. It has been discovered that a very high concentration pulse of VOC vapor emits from the membrane module at the start of a venting cycle, i.e., directly after rising tank pressure initiates flow through the membrane and venting commences at the end of a stagnant period. After a while, the concentration of VOC in the permeate/exhaust gas decreases to a steady state value in the expected manner. A significant quantity of VOC vapor is released to the atmosphere by the time the gas venting portion of the cycle stops. As a result, the time-averaged quantity of VOC compounds discharged to the air is still unacceptably high.
It is desirable to reduce overall emissions of VOC compounds below that which results from conventional separation membrane-based, fuel tank vapor recovery systems.
Accordingly, the present invention now provides a cyclic gas separation process for separating more preferentially permeable gas components from less preferentially permeable gas components of a feed gas mixture of such components, the process comprising the steps of
(a) providing a membrane module having a selectively gas permeable membrane for the more preferentially permeable gas components and the less preferentially permeable gas components,
(b) simultaneously (i) feeding the feed gas mixture to the module to contact the feed gas mixture with a first side of the membrane, (ii) discharging from the module in fluid communication with a second side of the membrane a permeate gas mixture enriched in the more preferentially permeable gas components, and (iii) withdrawing from the module in fluid communication with the first side of the membrane a retentate gas mixture enriched in the less preferentially permeable gas components,
(c) stopping the feeding of the gas feed mixture to the first side and stopping the discharging of the permeate gas mixture,
(d) charging a diluent gas to the module,
(e) ceasing the charging of diluent gas to the module, and
(f) repeating steps (b)-(e).
There is also provided a process for reducing atmospheric emissions of volatile organic compound vapor from ullage gas of a liquid volatile organic compound storage tank, the process comprising
(a) providing a vapor recovery system comprising (i) a membrane module comprising a two-sided gas permeable membrane comprising a polymer having a glass transition temperature and a selectivity for permeation of air relative to permeation of volatile organic compounds, and (ii) an exhaust pump having an intake and a discharge to ambient atmosphere, in which one side of the membrane defines a feed-retentate chamber on a first side of the membrane in fluid communication with the ullage gas and the second side of the membrane defines a permeate chamber in fluid communication with the intake of the exhaust pump,
(b) for a first length of time simultaneously and continuously (i) conveying the ullage gas into the feed-retentate chamber of the module so as to contact the first side of the membrane with the ullage gas, (ii) separating the ullage gas to form a low organic content gas depleted in volatile organic compounds relative to the ullage gas and a high organic content gas enriched in volatile organic compounds relative to the ullage gas, (iii) exhausting the low organic content gas from the module to ambient atmosphere, and (iv) returning the high organic content gas from the module into the ullage gas in the storage tank,
(c) for a second length of time, stopping the conveying of the ullage gas, the separating of the ullage gas, the exhausting of the low organic content gas and the returning of the high organic content gas,
(d) during step (c) charging air to the vapor recovery system in a quantity effective to remove volatile organic compounds from the membrane module, and
(e) repeating steps (b)-(d).
This invention also provides an improved vapor recovery system for controlling emissions of vapor from a storage tank containing volatile organic compound comprising (a) a storage tank containing an inventory of liquid of volatile organic compounds and an ullage within the tank containing a vapor of the volatile organic compounds, (b) a membrane module comprising a selectively gas permeable two sided membrane adapted to separate air from the volatile organic compounds and which membrane defines a feed-retentate chamber on the first side of the membrane and a permeate chamber on the second side of the membrane, (c) a blower adapted to move a gas mixture from the ullage into the feed-retentate chamber in contact with the membrane, (d) a return transfer line in fluid communication between the membrane module and the ullage and adapted to move a high organic content gas enriched in volatile organic compounds relative to the gas mixture back to the ullage, (e) a vent transfer line in fluid communication between the membrane module and ambient atmosphere, and (f) an exhaust pump in the vent transfer line which pump is operative to exhaust a low organic content gas depleted in volatile organic compounds relative to the gas mixture from the membrane module, wherein the improvement comprises an air charging means for intermittently introducing air into the membrane module.
The novel process and system are effective to reduce the cumulative amount per cycle of less preferentially permeable gas components in the permeate gas mixture below that which is produced when no air is charged into the membrane module.